It is known, from for example British Pat. No. 1,176,646 and U.S. Pat. No. 4131510, that radial dilation of the core of a nuclear reactor results in a decrease in core reactivity
U.S. Pat. No. 4,131,510 discloses an electromagnetically-operated core restraint structure which normally restrains radial dilation of the core but, in response to a temperature rise above the Curie point of the ferromagnetic materials employed, is disabled to permit radial dilation and thereby reduce reactivity.
British Patent No. 1,176,646 is concerned with a different approach in which the fuel sub-assemblies forming the core are mounted at their lower ends via ball joints seating in bearing cups fabricated from materials having differing coefficients of thermal expansion. In response to temperature increase at the coolant inlets of the sub-assemblies (which inlets are located below the core itself), the differing expansions lead to deflections of the sub-assemblies about the ball joints in directions corresponding to core dilation.
U.S. Pat. No. 3,600,276 discloses a reactivity control arrangement for a nuclear reactor core in which the fuel sub-assemblies contact one another through the agency of inclined contact faces acting in the manner of wedges, the fuel sub-assemblies being arranged in groups with differing mounting arrangements so that one group undergoes displacement relative to the other when the sub-assemblies undergo thermal expansion.
British Patent Applications Nos. 2,048,552A and 2,064,202A disclose grid designs for pressurised water reactor core fuel elements in which bimetallic components are incorporated in certain ones of the fuel element grids in order to control bowing of the fuel elements during reactor operation.
An object of the present invention is to provide improved reactivity control means for a nuclear reactor core.